The semiconductor substrates are preferably in the form of approximately round, disk-shaped substrates. The disk comprises a notch or at least one flat for denoting the orientation of the crystal, this thereby causing a deviation in the round disk shape of the substrate. The notch is in the form of a small nick that is produced in the edge of the semiconductor substrate (which is also referred to as a wafer), whilst the flat is in the form of a flattening of the edge of the wafer. In certain types of wafer, two such flattenings may also be formed, these being arranged at an angle with respect to one another. These days, semiconductor substrates of up to approximately 300 mm in diameter are used in the field of silicon technology.
During the many working steps utilised for the production of electronic circuits on a semiconductor substrate, the latter must also be positioned on rotation devices. Hereby, the rotation devices are preferably arranged within process chambers in order to enable the semiconductor substrate to be rotated during the processing step e.g. for the purposes of improving the outcome of the process with regard to uniformity across the cross section of the wafer. When using semiconductor substrates of 300 mm in diameter, the rotational speeds within the process chambers today amount to up to 150 revolutions per minute in dependence on the process being effected (e.g. in the case of rapid heating systems or so-called RTP plants). The semiconductor substrate is preferably placed on the rotation device by means of a robotic arm. Hereby, the semiconductor substrate preferably rests upon three quartz pins which are connected to the rotation device and form a tripod, said pins possibly being of different constructions in dependence on the process. Alternatively, the wafer is placed on a carrier ring, a so-called susceptor which is connected to the rotation device. Possible constructions of the quartz pins are described e.g. in the applicant's German patent application DE 100 03 639.2. Some further examples of holding devices for the semiconductor substrate such as susceptors for example are described in EP 0 821 403, U.S. Pat. Nos. 5,683,518 5,121,531 and 5,252,132.
In the face of increasing demands in regard to the precision and reproducibility of the outcome from the process, it is important to position the semiconductor substrate (the wafer) on the rotation device in as precise a manner as possible. These days, different methods and techniques are used in order to achieve this requirement whereby the wafer is generally transported and positioned by means of robots. If the robotic arm has no sensor technology, then the robot must be programmed manually so that it will place the wafer on the rotation device at the correct position and then remove it again. Hereby, wafers are positioned and removed by an operator in a series of test runs during which the control co-ordinates for the robot are changed until such time as the result corresponds to the requirements. This is very time-consuming especially in those plants where the operator does not have direct access to the rotation device, e.g. if the rotation device is surrounded by a process chamber.
Apart from manually programming the robot, systems are also in use wherein the surrounding environment is measured by means of a sensor arrangement in order to move the wafer into the desired position using the measuring data. Other systems scan this environment by means of the robotic arm or the end effecter (a wafer holding device attached to the robotic arm for holding the wafer whilst it is being transported by means of the robot) by moving it slowly up to defined edges or objects in order to detect the rise in value of the motor current when it touches them. The position of the robotic arm or of the end effecter is stored when a motor current threshold value is reached. Measurement of the robot's environment and adjustment of the robot's movement within a known environment are thereby possible thus dispensing with a manual adjustment.
The procedures for programming the robot that have been described above exhibit some disadvantages. Thus, for example, the error rate is very high in the case of manual programming and is substantially dependent on the experience of the operator. In the case of the automated systems which measure the environment by means of sensors or by the rise in the motor current of the robot, it is generally necessary to have access to the object, i.e. here, to the rotation device. This is not always possible in practice, e.g. if the object is located within a process chamber e.g. an RTP chamber. A further crucial disadvantage is the fact that only a static measurement of the robot's environment can be effected by means of the aforesaid procedure, i.e. the exact position of a rotational axis with respect to fixed points of reference, e.g. the chamber walls of a process chamber, cannot generally be determined thereby. In general, requires a determination of the rotational axis rotation of the rotation device and thus operation of the plant. However, the exact position of the rotational axis cannot be determined using the aforementioned static procedures since the rotation device is not usually permitted to be in operation during the measurement of the robot's environment. Consequently, the relative position between the wafer and the rotational axis is also indefinite, this being something that can lead to the process having disadvantageous results and could lead to disturbances when in continuous operation.